A spark plug used for ignition in an internal combustion engine, such as an automobile engine, is affected by a tendency of increasing combustion chamber temperature for the purpose of increasing engine output and improving fuel economy. In order to enhance ignition, in an increasing number of engines the discharge portion facing to a spark discharge gap of a spark plug projects into a combustion chamber. In such applications, a discharge portion of the spark plug is exposed to high temperature, and thus spark-effected wear of the discharge portion tends to accelerate. In order to enhance spark-effected wear resistance of a discharge portion facing to a spark discharge gap, there have been proposed many spark plugs of the type in which a noble-metal chip containing a predominant amount of Pt, Ir, or a like element is welded to the tip of an electrode.
For example, Japanese Patent Application Laid-Open (kokai) No. 9-7733 discloses a spark plug in which a noble-metal chip contains an alloy of Ir and Rh so as to utilize the merit of Ir; i.e., high melting point, and simultaneously prevent oxidational volatilization of Ir at high temperature (about 900° C. or higher), thereby enhancing wear resistance (spark resistance and high temperature oxidization are hereinafter referred to as “wear resistance” in the present specification) at higher temperature.
However, Rh is expensive several times as compared with Ir and moreover, the melting point of Rh is 1,970° C. and low in comparison with 2,454° C. of Ir. Therefore, if the Rh content is excessively increased, there arises a problem that not only the material cost of the noble-metal chip elevates highly but also the resistance against spark wear becomes insufficient.
To solve this problem, the present inventors have attempted to improve the spark wear resistance while restraining the wear due to oxidational volatilization by reducing the Rh content. However, it has been found that if the Rh content is reduced as such, rather the wear in the discharge portion (noble-metal chip) cannot be restrained in some cases due to generation of unusual wear which is described later.
More specifically, the present inventors conducted the following experiment. A 6-cylinder gasoline engine (piston displacement: 2,000 cc) was equipped with a spark plug configured such that only a center electrode has a discharge portion implemented by a noble-metal chip containing Ir as the main component and containing 20% by weight of Rh. The gasoline engine was operated at a speed of 5,000 rpm with the throttle opened completely and while using unleaded gasoline as fuel. After 20-hour operation, the appearance of the noble-metal chip was observed. As a result, as shown in FIG. 5, the noble-metal chip exhibited unusual wear, specifically, the noble-metal chip was arcuately eroded at a circumferential side surface which is not a discharge face (top surface of the discharge portion) facing the ground electrode. As is understood from FIG. 5, the unusual wear is unusual also in the form of wear and the cause for wear cannot be explained merely by spark discharge or oxidational volatilization. Although not shown, this unusual wear was similarly observed when the above-described operation (test) was performed using a noble-metal chip containing Ir as the main component and containing 10% by weight of Rh, 5% by weight of Rh or 1% by weight of Rh. As the Rh content becomes smaller, the degree of eroding from the circumferential side surface of the discharge portion became severer, in other words, unusual wear was more readily generated. As such, if a noble-metal chip increased in the Ir content and reduced in the expensive Rh content is used with an attempt to improve the spark wear resistance of the discharge portion and restrain the oxidational wear, unusual wear is newly incurred, failing in completely eliminating wear in the discharge portion.
The object of the present invention is to provide a spark plug having a discharge portion formed of a noble-metal chip, where the noble-metal chip forming the discharge part is increased in the Ir content and reduced in the expensive Rh content and which can be prevented from generation of unusual wear of eroding the circumferential side surface of the noble-metal chip and is equipped with a noble meal chip having excellent wear resistance.